International publication No. WO 90/06250 discloses an anti-lock control system which includes switching means to identify wheel and axle oscillations and intervening means to initiate measures for damping such oscillations. The rotational behavior of the wheels is measured by wheel sensors. In this publication, it is assessed as a criterion for oscillation recognition that the cycle times have approximately the same length when an oscillation prevails.
In a method disclosed in European patent application No. 0 262 602, the second time derivative of the speed of a drive wheel is compared to a reference value, and a signal indicative of drive train oscillations is emitted if the second time derivative exceeds the reference value.
Further, GB patent application No. 2 289 097 describes an anti-lock system wherein the rotational speed is determined on the differential from the speeds of the driven wheels and it is found out by filtering this signal whether an oscillation component which is in the frequency range of the drive train oscillations is superimposed on this rotational speed. Drive train oscillations prevail if two conditions are satisfied, i.e., if the oscillation frequency ranges between 5 and 12 hertz and the filtered signal exceeds a threshold value. A correction of braking pressure modulation which dampens the oscillations is performed.
An object of the present invention is to develop a reliable method which permits recognizing critical wheel or drive train oscillations that require countermeasures, i.e., damping measures, and avoiding unjustified control intervention. Another objective is to effectively damp the oscillations.
A signal is produced which can be evaluated as a basis for an effective damping of the drive train oscillations without jeopardizing the safety of the vehicle.
The present invention discloses a method wherein, upon the occurrence of oscillations with a frequency that lies within a critical range including the frequency range of typical drive train oscillations, the duration of the oscillating process on each individual wheel or the number of the successive oscillations or semi-wave oscillations is counted and, when a predetermined counter reading is reached, recognition of drive train oscillations is indicated if, at that time, the vehicle acceleration is above a predetermined limit value and if, at that time, oscillation recognition has not been triggered on the second wheel of the driven axle.
The special features of the method of the present invention for damping the oscillations include that when critical oscillations are recognized on a drive wheel selected according to predetermined criteria, the braking pressure control of this wheel is switched over for a defined interval to a special control which effects the oscillation damping operation.
Favorable embodiments of the method of the present invention are disclosed, for example, that the exceeding of a counter reading from 3 to 7, especially 4, successive semi-wave oscillations is set as a condition for the drive train oscillation recognition.
It has been found appropriate to predefine the limit value for the vehicle acceleration which must be exceeded for oscillation recognition by -0.6 g to -0.3 g or approximately -0.5 g. It is ensured thereby that the oscillation recognition according to the present invention occurs only in the range of low coefficients of friction.
In a preferred aspect of the method of damping the oscillations, the commencement of the special control prevents a braking pressure reduction on the wheel concerned, and a braking pressure increase induced by the control is allowed, if at all, delayed and/or with a flat or flattened gradient only. It may also be suitable to maintain the braking pressure at least roughly constant during the special control.
The special control will not be terminated until the brake slip of the wheel exceeds a predetermined high limit value in the order ranging between 40% and 60%, for example, 50%.
According to the present invention, wheel oscillations are assessed as critical which lie in a predetermined frequency range of e.g. between 5 hertz and 23 hertz typical of drive train oscillations, which occur at a vehicle acceleration that exceeds a limit value and which last longer than a predetermined period of time or number of oscillations.
It is an objective to react very quickly to wheel oscillations or drive train oscillations, however, an unjustified intervention must be avoided. In an embodiment, the minimum number of five successive semi-wave oscillations is assessed as a criterion for the change-over to the special control or the initiation of damping measures.